1. Field of the Invention
This invention relates to a method for making thermal heads. More particularly, the present invention relates to a method for making a plurality of thermal heads from a material substrate board.
2. Description of the Prior Art
As is well known, thermal heads are widely used in facsimile machines to print transmitted information on thermosensitive paper. The thermal head is also used in printers of the type wherein the ink of a transfer ink ribbon or film is thermally caused to be transferred onto printing paper.
There are various types of thermal heads which include line-type heads and matrix-type heads. The line-type thermal head has a row (line) of multiple heating dots, as disclosed for example in Japanese Patent Application Laid-open No. 63-221055. The matrix-type thermal head has a multiplicity of heating dots arranged in a matrix, as disclosed for example in U.S. Pat. No. 3,855,448 to Hanagata et al.
The present invention is applicable to both types of thermal heads as long as the head incorporates a substrate which carries a common electrode along and adjacent to a marginal edge of the substrate. FIG. 1 of the accompanying drawings illustrates a typical line-type thermal head to which the present invention is advantageously applied.
As shown in FIG. 1, the typical line-type thermal head comprises an elongate insulating substrate 1 having a length L and a width S. The substrate 1 has one surface formed with a conductor pattern which includes a common electrode 31 extending along and adjacent to one longitudinal edge, individual electrodes 32 and other circuit parts (not shown). The common electrode 31 has a pair of side portions 31a extending along the two shorter edges of the substrate. The substrate further carries a heating resistor line 4 electrically connected to the common electrode 31 and the individual electrodes 32, and an array of drive IC's 5 connected to the individual electrodes 32. The drive IC's cause divisional heating of the resistor line to provide a line of heating dots.
In manufacturing such a thermal head, use is made of a rectangular material substrate board 2, as shown in FIGS. 5 and 6. Specifically, the substrate board 2 has a size enough to provide a plurality of substrates, and is formed with a conductor layer 3. The conductor layer 3 has bulging portions 3a of a width S1 along the four sides 2a-2d of the substrate board 2 because a conductor paste (which is subsequently dried and cured) for the layer 3 has a tendency of bulging at edged positions due to the surface tension of the paste.
Obviously, the bulging portions 3a of the conductor layer 3 are not suitable for etching a sophisticated circuit portion because insufficient conductor removal tends to occur at the thicker bulging portions 3a. This problem becomes more remarkable when the substrate board 2 has a glazing layer (not shown) which itself has a tendency of bulging at the four sides of the substrate board.
Therefore, in a prior art method for making thermal heads from such a substrate board 2, the conductor layer 3 is etched to provide conductor patterns only in regions (e.g. three regions for providing three thermal heads A) which are located clear of the bulging portions 3a, as shown in FIG. 7. Thus, the bulging portions 3a at the four sides are left blank and later removed away at the time of separating the thermal heads A by cutting or cracking (cracking along a scribed line or pre-formed groove).
The prior art method described above is disadvantageous in the following respects.
First, the substrate board 2 must have an overall width So which is at least equal to a multiple of the individual substrate width S plus double the width S1 of the bulging portions 3a, and all of the bulging portions must be later removed for separating the thermal heads A. Thus, waste of material is unavoidable according to the prior art method.
Secondly, an operation of removing the bulging portions 3a by cutting or cracking is required in addition to separating the thermal heads A from each other. Combined with the material waste, such an additional operation increases the production cost. Further, the additional cutting or cracking increases the possibility of damaging the intricate conductor pattern of each thermal head, thus decreasing the yield.